1. Field of the Invention
The present invention relates to a process for producing 3-chloro-5-trifluoromethylpyridine derivatives by reacting 5-trifluoromethylpyridine derivatives with chlorine in a vapour phase in the presence of a catalyst.
2. Description of the Prior Art
Heretofore, it has been difficult to produce 5-trifluoromethylpyridine derivatives having a chlorine atom at 3-position of the pyridine ring. Particularly, it has been extremely difficult to produce the derivative having a chlorine atom at 2- and 3-positions and a trifluoromethyl group at 5-position, i.e. 2,3-dichloro-5-trifluoromethylpyridine.
Meantime, this 2,3-dichloro-5-trifluoromethylpyridine has been increasingly in demand in recent years as it is useful as an intermediate for an active ingredient for various agricultural chemicals. Therefore, it is desired to develop an industrially advantageous process for its production.
The present applicant has proposed certain processes for its production in Japanese Unexamined Patent Publications 90059/81, 97271/81 and 125369/81. However, these processes are all concerned with a liquid phase reaction. In the first two processes, 2,3-dichloro-5-trifluoromethylpyridine is produced e.g. by reacting 2-chloro-5-trifluoromethylpyridine with an aqueous ammonium solution under elevated pressure to form 2-amino-5-trifluoromethylpyridine, which is then chlorinated to form 2-amino-3-chloro-5-trifluoromethylpyridine, which is further subjected to diazotization-chlorination. These processes require a series of reaction steps, and it is necessary to conduct the reaction under elevated pressure. Besides, they involve difficulties in the treatment of the waste water. Whereas, the process disclosed in Japanese Unexamined Patent Publication 125369 is concerned with the production of 2,3-dichloro-5-trifluoromethylpyridine by reacting 2-chloro-5-trifluoromethylpyridine with chlorine in a liquid phase in the presence of a certain metal chloride as a catalyst. However, this process is economically disadvantageous in that it requires a great amount of the catalyst, for example, as much as the same amount by weight as the pyridine starting material. Further, the after-treatment of the reaction product involves difficulties, and the reaction time is rather long, for example, as long as from 15 to 20 hours.
On the other hand, it is known to chlorinate .beta.-picoline or .beta.-trifluoromethylpyridine in a vapour phase at an elevated temperature. However, such a process is concerned with chlorination of .alpha.-position of the pyridine ring by a radical reaction and not with chlorination of .beta.-position. For example, U.S. Pat. No. 4,241,213 and others disclose a process for chlorination of .beta.-picoline in a vapour phase at a temperature of from 300.degree. to 500.degree. C., whereby a mixture of products is obtainable which is composed mainly of those chlorinated at the side chain methyl group and .alpha.-position such as 2-chloro-5-trichloromethylpyridine and 2,6-dichloro-5-trichloromethylpyridine. U.S. Pat. No. 4,288,599 discloses a process for chlorinating and fluorinating .beta.-picoline in a vapour phase at a temperature of from 300.degree. to 600.degree. C., whereby a mixture of products is obtainable which is composed mainly of those fluorinated at the side chain methyl group and chlorinated at .alpha.-position such as 2-chloro-5-trifluoromethylpyridine and 2,6-dichloro-5-trifluoromethylpyridine. Further, the publication of European Patent Application 13474 discloses a process for chlorinating .beta.-trifluoromethylpyridine in a vapour phase at a temperature of from 300.degree. to 450.degree. C., whereby a mixture of products is obtainable which is composed mainly of .alpha.-chlorinated products such as 2-chloro-, 6-chloro- and 2,6-dichloro-5-trifluoromethylpyridines.